The present invention relates to a projection exposure system, and more particularly, to a modified illumination system for a projection exposure system.
Known projection exposing technology for forming fine patterns such as semiconductor devices include an excimer laser projection exposure system, a phase shift mask method, and a tilted illumination method. The tilted illumination method can improve resolution without adding new equipment, and therefore is widely used. Also, an annular illumination method using lenses with annular apertures can improve resolution by 15-20% and double depth of focus.
FIG. 1 illustrates schematically an illumination system for a conventional projection exposure system. Referring to FIG. 1, a reflecting mirror 2 is placed in front of light source 1. A parallel-light-forming lens 3 and a fly's eyes lens 4 are sequentially located along a light traveling path at the back of light source 1. In light emitted from light source 1 is reflected and converged by reflecting mirror 2, to be incident on parallel-light forming lens 3. Light which is nearly parallel is obtained from parallel-light forming lens 3 and focussed on fly's eyes lens 4. In a cross-section taken along line A--A, the light intensity just prior to incidence on fly's eyes lens 4 is great at the center and weak around the periphery. The light intensity is shown as plot (a) in FIG. 4.
However, when using annular illumination or tilted illumination, which utilize only peripheral light, since the amount of the peripheral light is small, the light utilizing efficiency is very low. Accordingly, the exposure time is lengthened, which thereby lowers productivity.